In order to realize high performance organic light-emitting devices (OLEDs), OLEDs should achieve high external quantum efficiency (EQE) and low operating voltages at the same time. The theoretical limit of operating voltage is generally believed to be equal to the energy gap (Eg), which is corresponding to the energy difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) for the emitter molecule divided by the electron charge, e. Here, we demonstrate OLEDs operating below a theoretical limit of the Eg voltage with high EQE of 30%. We used a phenylpridine-based electron-trasporter and a green phosphorescent emitter, fac-tris(2-phenylpyridine)iridium(III) with a peak emission wavelength of 523 nm, which is equivalent to a photon energy of 2.38 eV. An optimized green OLED operated clearly below the theoretical limit of the Eg voltage at 2.38 V showing 100 cd m–2 at 2.25 V and 5000 cd m–2 at 2.95 V without any light outcoupling enchancement techniques. Further, we used a homoleptic iridium (III) tris(phenylimidazolinate) complex as a blue phosphorescent emitter, which showed a peak emission wavelength of 476 nm (2.62 eV). By careful optimization, we developed a blue device with high EQE of 30% and a low turn-on voltage of 2.5 V at 1 cd m–2. This device also showed high power efficiency over 75 lm W–1 and ideal light distribution pattern at 100 cd m–2. [1] H. Sasabe, J. Kido, J. Mater. Chem. C 2013, 1, 1699. [2] H. Sasabe, J. Kido et al., Adv. Funct. Mater. 2013, 23, 5550. [3] H. Sasabe, J. Kido et al., Adv. Mater. 2014, 26, 5062.